The fraction of gas in a two component flow of liquid and gas may be defined as the volume of gas in an arbitrary section of a tube divided by the volume of that section of the tube. The fraction of gas may therefore be expressed as a number varying from zero (when the flow is entirely comprised by the liquid) to one (when the flow solely consists of gas).
While it has been of great importance to know how much of the production from an oil well is comprised by oil and how much of it is gas, measurement of the fraction of gas has always been considered with great interest within the offshore oil business.
Measuring devices whose operating principle is based on the detection of changes in capacitance, have been increasingly employmed for the measurement of the fraction of gas in two component fluid comprising liquid/gas mixtures.
The principle for such a measuring method is well known and amounts essentially to measuring the electrical capacitance across two electrodes, between which the mixture of the two components is flowing. If the area and the mutual separation of the electrodes are fixed, the measured capacitance will be related to the fraction of gas in the mixture between the electrodes.
By employing such a technique, it is in principle possible to build instruments with rapid dynamic response. The instrument may be constructed in such a way as to give a non-intrusive method of measurement. In spite of the advantage which is achievable in this manner, commercially available gas fraction meters based on the capacitance principle are relatively scarce.
The main disadvantage of known capacitance gas fraction meters is the dependence of the instrument calibration upon the nature of the flow regime being monitored. Thus, by example, the calibration curve required for a bubble flow will deviate from that of an stratified flow.
One of the reasons for this flow regime dependency is the missing homogenity of the electric field through the sample volume. If the electric field varies through the whole sample volume, the measured change in capacity which arises from a change in the gas fraction, will depend upon where the gas is located.